Polycrystalline silicon rod and process for production thereof

ABSTRACT

The invention provides a polycrystalline silicon rod having a total diameter of at least 150 mm, including a core A having a porosity of 0 to less than 0.01 around a thin rod, and at least two subsequent regions B and C which differ in porosity by a factor of 1.7 to 23, the outer region C being less porous than region B.

BACKGROUND OF THE INVENTION

The invention relates to a polycrystalline silicon rod and to a processfor production of a polycrystalline silicon rod.

Polycrystalline silicon (polysilicon for short) serves as a startingmaterial for production of monocrystalline silicon for semiconductors bythe Czochralski (CZ) or zone-melting (FZ) processes, and for productionof mono- or polycrystalline silicon by various drawing and castingprocesses for production of solar cells for photovoltaics.

Polycrystalline silicon is generally produced by means of the Siemensprocess.

In this process, in a bell-shaped reactor (“Siemens reactor”), supportbodies, typically thin filament rods (thin rods) of silicon, are heatedby a direct passage of current, and a reaction gas comprising hydrogenand one or more silicon-containing components is introduced.

Typically, the silicon-containing component used is trichlorosilane(SiHCl₃, TCS) or a mixture of trichlorosilane with dichlorosilane(SiH₂Cl₂, DCS) and/or with tetrachlorosilane (SiCl₄, STC). The use ofmonosilane (SiH₄) is also known.

The thin rods are typically inserted vertically into electrodes presentat the reactor base, through which they are connected to the powersupply. Every two thin rods are coupled via a horizontal bridge(likewise made from silicon) and form a support body for the silicondeposition. The bridge coupling produces the typical U shape of thesupport bodies.

High-purity polysilicon is deposited on the heated thin rods and thehorizontal bridge, as a result of which the diameter thereof grows withtime.

The deposition process is typically controlled by the setting of rodtemperature and reaction gas flow rate and composition.

The rod temperature is measured with radiation pyrometers, usually onthe surfaces of the rods facing the reactor wall.

The rod temperature is set by control or regulation of the electricalpower, either in a fixed manner or as a function of the rod diameter.

The amount and composition of the reaction gas are set as a function ofthe time or rod diameter.

After a desired diameter has been attained, the deposition is ended andthe polysilicon rods which have formed are cooled to room temperature.

The morphology of the growing rod is determined by the parameters of thedeposition process.

The morphology of the deposited rods may vary from compact and smooth upto very porous and fissured material.

U.S. Pat. No. 6,350,313 B2 discloses the further processing of compactpolycrystalline silicon rods.

Compact polycrystalline silicon is very substantially free of cracks,pores, gaps, fissures etc.

The apparent density of such a material corresponds to the true densityof polycrystalline silicon and is 2.329 g/cm³.

US 2003/0150378 A2 discloses “teardrop poly” and a process forproduction thereof. In this process, a compact, hole-free, high-puritypolysilicon rod is deposited from monosilane SiH₄ by means of theSiemens process up to a silicon rod diameter of 45 mm at 850° C. and asilane concentration of 1.14 mol %. Subsequently, the rod surfacetemperature is increased instantly from 850 to 988° C. and the silaneconcentration reduced instantly from 1.14 to 0.15 mol %. This parameterjump instantly alters the growth of the silicon crystals on the siliconrod, and needles, called dendrites, grow out of the rod surface. Thesedendrites can subsequently be removed from the compact rod part, whilethe compact part has to be processed further separately.

US 2010/219380 A1, in contrast, discloses a polycrystalline silicon rodhaving an apparent density in the range from 2.0 to 2.3 g/cm³ and anoverall porosity of 0.01 to 0.2. The silicon rod has a similarstructure, though this structure contains pores, gaps, crevices, cracksand fissures. Such a polycrystalline silicon rod can be comminuted intochunks with comparatively low energy expenditure, and accordingly leadsto less surface contamination at the surface of the chunks.

US 2010/219380 A1 likewise discloses a process for producing apolysilicon rod as claimed in any of claims 1 to 3, in which a stream ofa reaction gas comprising a chlorosilane mixture and hydrogen isintroduced into a reactor and high-purity polysilicon is deposited on afilament rod of silicon heated by direct passage of current, thefilament rod being formed from two vertical rods and one horizontal rod,and the horizontal rod forming a linking bridge between the verticalrods, characterized in that the chlorosilane mixture used is a mixtureof di- and trichlorosilane and the passage of current through thefilament rod is regulated such that the filament rod has a temperatureat the underside of the bridge between 1300 and 1413° C. and thetemperature of the reaction gases in the reactor is measured andadjusted so as not to be more than 650° C., and the flow rate of thechlorosilane mixture is adjusted to its maximum value within less than30 hours, preferably within less than 5 hours, from the start of supplyof the chlorosilane mixture.

The compact rods are more expensive to produce. The deposition processis slower. However, compact rods generally lead to better yields insubsequent crystallization steps.

The increase in the base parameters of rod temperature, specific flowrate, silane concentration generally leads to an increase in thedeposition rate and hence to an improvement in the economic viability ofthe deposition process.

However, natural limits are set on each of these parameters, theexceedance of which disrupts the production process.

If, for example, the concentration of the silicon-containing componentselected is too high, there may be homogeneous gas phase deposition.

The effect of an excessively high rod temperature may be that themorphology of the silicon rods to be deposited is not compact enough toprovide a sufficient cross-sectional area to the current flow, whichrises with the growing rod diameter. If the current density becomes toohigh, this can cause the melting of silicon. From a certain diameter ofabout 120 mm, even in the case of compact morphology, silicon in the rodinterior can become liquid, since high temperature differences existbetween surface and rod center.

This is also problematic in the process according to US 2003/0150378 A2,since the current flows exclusively through the compact part of thesilicon rod. If the diameter of the compact part selected is too low,which is actually desirable since the aim of the process is theproduction of dendrites, there is a risk of melting. With risingdiameter, higher currents are required, and so the diameter of thecompact part must also increase. This reduces the yield of dendrites.

In the case of a polycrystalline silicon rod according to US 2010/219380A1, in contrast, a majority of the rod cross section is available forcurrent flow. The electrical conductivity is not impaired by the smallcracks and pores compared to conventional compact silicon.

For most applications, polycrystalline silicon rods have to be crushedinto smaller chunks. Typically, the chunks are subsequently classifiedby size. A process for comminuting and sorting polysilicon is described,for example, in U.S. Pat. No. 8,074,905 B2. In general, it is immaterialhere whether the polycrystalline silicon is in compact or brittle form.

The morphology of polycrystalline rods and of chunks obtained therefrom,however, has a strong influence on the performance of the product.

As mentioned above, compact rods show better yields in crystal pulling.

A porous and fissured morphology like that according to US 2010/219380A1, in contrast, has adverse effects on the crystallizationcharacteristics. This particularly affects the demanding CZ process, inwhich it has not been possible to date to use porous and fissured chunksowing to the economically unacceptable yields.

U.S. Pat. No. 7,939,173 B2 discloses a polysilicon rod which, in theradial cross section, has regions with different crystal structures, aninner structure comprising few acicular crystals, if any, and an outerstructure comprising acicular crystals and microcrystals, with presenceof a mixed zone in which there is a fluid transition from the innerstructure to the outer structure. This polysilicon rod is intended foruse in the FZ process. The production is effected by deposition ofsilicon from hydrogen-diluted chlorosilanes having a molar proportion ofthe chlorosilanes of not more than 30% on a filament rod of silicon at arod temperature of 950 to 1090° C. at the start of deposition. To obtainthe different crystal structures, the process parameters are altered ina fluid manner. The rod temperature is lowered and the amount ofhydrogen injected reduced, such that the molar proportion of thechlorosilanes is increased to 35-60%.

The problem described gave rise to the objective of the presentinvention.

The aim was to provide polycrystalline silicon which is less expensiveto produce than compact material but nevertheless exhibits goodperformance in CZ crystal pulling.

DESCRIPTION OF THE INVENTION

The object of the invention is achieved by a polycrystalline silicon rodhaving a diameter of at least 150 mm, comprising a core (A) having aporosity of 0 to less than 0.01 around a thin rod, and at least twosubsequent regions B and C which differ in porosity by a factor of 1.7to 23, the outer region C being less porous than region B.

Core A preferably extends over a diameter region of up to 60 mm. Thethin rod on which core A is deposited typically has an extent of a fewmm up to 12 mm. Thus, core A typically starts, for example, at adiameter of 9 mm and extends up to a diameter of not more than 60 mm.Core A preferably extends up to a diameter of not more than 50 mm, morepreferably not more than 40 mm.

Preferably, region B which follows core A has the greatest porosity of0.06 to 0.23 and extends over a diameter region of 15% to 90% of thediameter of the silicon rod. Region B preferably extends over a diameterregion of 20-80%.

In the case of a diameter of the silicon rod of 150 mm, region Bpreferably extends over a region of a diameter of at least 22 mm up to adiameter of not more than 145 mm, more preferably over a region of45-120 mm.

In the case of a diameter of the silicon rod of 200 mm, region Bpreferably extends over a region of a diameter of at least 30 mm up to adiameter of not more than 180 mm, more preferably over a region of40-160 mm.

The subsequent region C preferably has a lower porosity of 0.01 to 0.1and extends over a diameter region of 50% to 100% of the total diameterof the silicon rod. Region C preferably extends over a diameter regionof 60-100%, more preferably over a region of 70-100%.

In the case of a diameter of the silicon rod of 150 mm, region Cpreferably extends over a region of a diameter of at least 75 mm up to adiameter of not more than 150 mm, more preferably over a region of90-150 mm, most preferably over a region of 105-150 mm.

In the case of a diameter of the silicon rod of 200 mm, region Cpreferably extends over a region of a diameter of at least 100 mm up toa diameter of not more than 200 mm, more preferably 120-200 mm, mostpreferably 140-200 mm.

The porosity in region C is preferably constant. It is especiallypreferable when the porosity in region C decreases with increasingdiameter.

It is additionally preferable when a final layer Z applied to the porousregions B and C has a porosity of 0 to less than 0.01 (compact) within adiameter region of 90% to 100% of the total diameter. A particularlypreferred diameter region is 95-100%.

In the case of a diameter of the silicon rod of 150 mm, layer Zpreferably extends over a region of a diameter of at least 135 mm up to150 mm.

In the case of a diameter of the silicon rod of 200 mm, layer Zpreferably extends over a region of a diameter of at least 180 mm up to200 mm.

The Z layer preferably has a thickness of at least 7.5 mm.

Particularly in the case of relatively low diameters, layer Z isadvantageous when layer C is not very thick, such that a very compactfinal layer smooths the surface.

A region of the silicon rod having a porosity of less than 0.01 isregarded as compact in the context of the invention. A region having aporosity of 0.01 to 0.1 is referred to as “impervious material” or“impervious layer”. Region C comprises impervious material.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is also illustrated hereinafter by figures.

FIG. 1 shows a schematic of the structure of an inventive silicon rodwhich additionally comprises a Z layer.

FIG. 2 shows a photograph of a crushed silicon rod.

A shows the compact core region A around the thin rod. B represents thefirst porous layer, C the impervious layer. Z denotes the optionalcompact layer Z.

The extent of the regions B and C is defined as a function of thediameter D of the silicon rod.

The total diameter of the polycrystalline silicon rod is at least 150mm.

The polycrystalline silicon rod preferably has a total diameter of atleast 180 mm.

Particular preference is given to a polycrystalline silicon rod having adiameter of at least 200 mm.

In FIG. 2, a compact core, a porous region and a compact outer regionare identifiable.

By comminuting the inventive polycrystalline silicon rod, it is possibleto produce polycrystalline silicon chunks.

The comminution of the rods is preferably effected analogously to EP 2423 163 A1 with subsequent dust removal from the chunks by means ofcompressed air or dry ice.

It is equally preferred, analogously to U.S. Pat. No. 8,074,905, tocrush the rods into chunks, to classify or to sort them into chunks ofsize class from about 0.5 mm to greater than 45 mm, and then to subjectthem to a wet-chemical purification—as described in EP 0 905 796 B1.

It is a feature of the amount of polycrystalline silicon chunks obtainedthat it includes chunks with different porosities and chunks whichcomprise regions with different porosities.

Additionally present are chunks comprising an outer face having a radiusof curvature of at least 75 mm.

The porosities of individual chunks vary preferably from 0 to 0.25.

Individual chunks have a porosity of 0 to less than 0.01 and originatefrom the compact core of the silicon rod or from the optionally presentZ layer.

Other chunks are more or less porous and have porosities of 0.01 to0.25.

Preference is given to the presence of chunks comprising an outer facehaving a radius of curvature of at least 90 mm.

Particular preference is given to the presence of chunks comprising anouter face having a radius of curvature of at least 100 mm.

The overall porosity of a sample is composed of the sum total of thecavities which are connected to one another and to the environment, andthe cavities which are not connected to one another. The overallporosity, i.e. the proportion of the total pore volume (open and closedpores) in the total volume of the polysilicon is determined to DIN-EN1936 from the calculation of apparent and true density, i.e. overallporosity=1−(apparent density/2.329 [g/cm³]).

The apparent density is defined as the density of the polysiliconincluding the pore space in the dry state to DIN-EN 1936 (weighing oftest specimens of defined volume or measurement of the buoyancy of thesaturated sample in mercury with a hydrostatic balance).

The compact core A of the polycrystalline silicon rod preferably has anapparent density of 2.329 (porosity 0).

Region B preferably has an apparent density of 1.8 to 2.2.

Region C preferably has an apparent density of 2.1 to 2.3.

Layer Z preferably has an apparent density of 2.25 to 2.329.

The object of the invention is also achieved by a process for producingpolycrystalline silicon rods by introducing a reaction gas comprising asilicon-containing component into a reactor, as a result of whichpolycrystalline silicon is deposited on thin rods up to a target roddiameter, which comprises

-   (a) in a first step, depositing a core (A) having a porosity of 0 to    less than 0.01 onto each of the thin rods up to a rod diameter of    15-60 mm, the rod temperature being 1000° C. to 1150° C., the    concentration of the silicon-containing component in the reaction    gas being 20 to 60 mol % and the feed rate of the silicon-containing    component being 100 to 550 kg/h per 1 m² of rod surface area;-   (b) in a second step, beginning at a rod diameter of at least 10% up    to a rod diameter of at most 90% of the target rod diameter,    depositing a layer (B) having a porosity of 0.06 to 0.23 onto core    (A), the rod temperature being 1030° C. to 1130° C., the    concentration of the silicon-containing component in the reaction    gas being 20 to 40 mol % and the feed rate of the silicon-containing    component being 80 to 200 kg/h per 1 m² of rod surface area;-   (c) and, in a third step, beginning at a rod diameter of at least    50% up to a rod diameter of at most 100% of the target rod diameter,    depositing a layer (C) having a porosity of 0.01 to 0.1 onto layer    (B), the rod temperature being 960° C. to 1030° C. and being at    least 20° C. lower than the rod temperature during the second step,    the concentration of the silicon-containing component in the    reaction gas being 15 to 35 mol %, and the feed rate of the    silicon-containing component being 10 to 130 kg/h per 1 m² of rod    surface area.

Preference is given to effecting a fourth step, beginning at a roddiameter of at least 90% up to a rod diameter of at most 100% of thetarget rod diameter, in which a layer (Z) having a porosity of 0 to 0.01is deposited onto layer (C), the rod temperature being 930° C. to 1000°C. and being at least 20° C. lower than the rod temperature during thethird step, the concentration of the silicon-containing component in thereaction gas being 3 to 30 mol %, and the feed rate of thesilicon-containing component being 6 to 60 kg/h per 1 m² of rod surfacearea.

The target rod diameter is preferably at least 150 mm, more preferablyat least 180 mm and most preferably at least 200 mm.

The porosities of the regions deposited are controlled by a suitableselection of rod temperature and concentration of the silicon-containingcomponent.

The feed rate of the silicon-containing component is preferably reducedin the course of the process. Preference is given to reducing the feedrate of the silicon-containing component in each of the individualprocess steps within the limits claimed.

Preference is given to increasing the concentration of thesilicon-containing component during step a). In step b) up to the end ofthe process, the concentration of the silicon-containing component ispreferably lowered continuously.

The transitions between the individual process steps are preferablyfluid. There are preferably no abrupt transitions. Process conditions atthe start of a process step correspond to the process conditions at theend of the preceding process step.

The silicon-containing component is preferably a chlorosilane.Preference is given to using hydrogen as the carrier gas.

Particular preference is given to the use of trichlorosilane.

The concentration of the chlorosilane in the hydrogen carrier gas isregulated by the feed rate of chlorosilane and the feed rate ofhydrogen.

Preference is given to continuously reducing the feed rate of hydrogenin the course of the process. Preference is given to reducing the feedrate of hydrogen in each of the individual process steps.

In the course of performance of the process, process parameters have tobe monitored. This is preferably effected as follows:

Gas flow rates are determined with commercial meters. The H₂ flow rateis measured by means of a volume flow rate meter (for example animpeller meter).

The flow rate of the chlorosilanes is determined by means of mass flowrate meters.

The temperature is measured with a commercial radiation pyrometer whichmeasures the surface temperature of the closest rod at a rod height ofabout 1 meter.

The rod diameter is monitored with an optical measurement method (forexample spyglass with calibrated division scale, camera, etc.) whichmeasures the diameter of a rod or determines the diameter from the rodseparations at a rod height of about 1 meter.

The inventive polycrystalline silicon rod comprises an innermost layeror a core region A around the thin rod which has been deposited rapidlyand is compact, i.e. by definition has a porosity of less than 0.01. Inthe deposition of the compact region, a comparatively low temperatureand a high gas pulse flow rate are selected (gas pulse flow rate=massflow rate*exit velocity at the nozzle).

The core region is followed by a first porous region B having a porosityof 0.06 to 0.23. This too has been deposited rapidly. This region Bcomprises, for example, holes, gaps and fissures. The porosity isdetermined essentially by temperature and gas flow rate.

The first porous region B is followed by a second porous region C whichhas a much lower porosity of 0.01 to 0.1 than region B. The rodtemperature during the production of region C is lower. Accordingly,this region C has been deposited more slowly than region B. Region Ccloses holes and scars. The surface is preferably flattened out.

The polycrystalline silicon rod preferably comprises a final layer tofinish the surface of the silicon rod and to bring about additionalsmoothing of the surface. This layer is compact.

The advantages of the invention lie particularly in a more favorableproduction process, especially since the predominant regions of thesilicon rod (core A and region B) are deposited rapidly. For instance,the deposition rates (diameter growth) for core A are preferably 1.5 toa maximum of 2 mm/h, and, for region B, 1.8 to less than 2.2 mm/h.

The disadvantages which are otherwise possessed by porous material andwhich are manifested particularly in poor performance in the course ofcrystal pulling are avoided by surface finishing by slower deposition ofnot very porous or compact outer regions. For instance, the depositionrates (diameter growth) for region C are preferably 1 to less than 1.5mm/h, and, for layer Z, 0.7 to less than 1.1 mm/h.

The production process is particularly economically viable since it isassociated with lower throughputs of reactant material and lower energyconsumption. Process disruption resulting from cracks, splinters andplant shutdowns does not occur.

The polycrystalline silicon rod shows a high yield in crystal pulling.

For instance, a mean dislocation-free rod length corresponding to thatof compact deposited silicon is achievable.

The invention is illustrated in detail by examples.

It should be mentioned that the process parameters to be selected dependon the reactor type selected. More particularly, the number of rods, theseparation of the electrodes and the nozzle diameter influence theprocess conditions. The examples which follow relate to an 8-rodreactor. In the case of larger reactors, for example a 48-rod reactor,the process parameters should be adjusted. On the basis of a fewexploratory experiments, it is possible for the person skilled in theart to apply the concept of the invention to all possible types ofSiemens reactors.

In the case of use of alternative silicon-containing components too—theexamples relate to deposition with trichlorosilane and hydrogen as thecarrier gas corresponding adjustments should be undertaken by the personskilled in the art.

EXAMPLES Comparative Example

For the comparative example, a deposition plant having 8 rods and havingan electrode separation of 270 mm was used. The total rod length (withbridges) was 20 280 mm.

The nozzle diameter was 16 mm. The reaction gas used was a mixture oftrichlorosilane and hydrogen.

Table 1 shows the process parameters selected for the comparativeexample. Two regions A and B were deposited. D in mm indicates thegrowing diameter of the silicon rod. T denotes the rod temperature in °C. Also reported are the feed rate of hydrogen and trichlorosilane andthe concentration of trichlorosilane in mol %.

TABLE 1 H₂ [m³ (STP)/ TCS [kg/ Conc. Region D [mm] T [° C.] h/m²] h/m²][mol %] A  9 1070 184 368 25 A  58 1070  51 150 33 B  68 1080  41 144 36B 100 1080  40 130 35 B 130 1080  36 108 33 B 160 1068  28  80 32 B 1801056  24  66 31

Region A (core region) extended up to diameter 58 mm and was depositedwith a temperature of 1070° C.

The amount of chlorosilane per unit silicon area was 368 to 150 kg/h/m²with a molar concentration in the carrier gas (H₂) of 25 to 33%.

The core region exhibited compact morphology without holes orinclusions. The porosity was less than 0.01.

Between 58 mm and 68 mm, there is a fluid transition from region A toregion B.

Region B followed on from layer A with a diameter of 68 to 180 mm at atemperature of 1080° C. to 1056° C.

The amount of chlorosilane per unit silicon area was 144 to 66 kg/h/m²with a molar concentration in the carrier gas (H₂) of 36 to 31%.

Region B showed a highly porous morphology with holes and gaps. Theporosity was 0.13.

The above-described rods were, as described in EP 2 423 163 A1, crushedinto chunks, sorted into chunks of size class from about 0.5 mm togreater than 45 mm, and treated by means of compressed air in order toremove silicon dust from the chunks. There was no chemical wet cleaningof the chunks.

The surface metal contamination in the polycrystalline silicon and thesize distribution of the individual chunk fractions corresponded to thevalues reported in EP 2 423 163 A1.

Owing to the different porosity of the rod, chunks with differentporosities were present.

A sample comprising 20 chunks of different size was selected randomlyand analyzed for porosity and (if present) with regard to the radius ofcurvature of the outer face thereof.

The result of the measurements is shown in Table 2.

The polysilicon chunks from the comparative example were also examinedwith respect to the performance thereof in crystal pulling.

For 20 rods produced according to the comparative example, a meandislocation-free length of 84% was found.

The dislocation-free length is defined from dislocation-free rod lengthwith respect to the maximum possible cylindrical utilizable rod length.

TABLE 2 Radius of curvature of the Chunk Mass [g] Porosity outer face(if present) [mm] #1   92.6 0.11 — #2  199.3 0.16 — #3  208.0 0.10 — #4  94.7 0.11 — #5  166.4 0.19 89.1  #6  206.0 0.20 90.25 #7  104.6 0.10 —#8  170.0 0.12 — #9   78.0 0.01 — #10 163.0 0.06 — #11 201.2 0.16 89.5#12 207.6 0.09 — #13 120.1 0.07 — #14  97.7 0.00 — #15 172.5 0.16 — #16187.5 0.13 — #17 104.2 0.09 — #18 157.1 0.09 — #19 163.4 0.21 90.95 #20159.0 0.09 —

Example 1

For the first inventive example, a deposition plant as specified in thecomparative example was likewise utilized.

As in the comparative example, deposition was effected withtrichlorosilane and hydrogen.

In a departure from the comparative example, in accordance with theinvention, an additional region C (less porous than B) and an optional Zlayer (compact) were deposited.

Table 3 shows the process parameters used.

TABLE 3 H₂ [m³ (STP)/ TCS [kg/ Conc. Region D [mm] T [° C.] h/m²] h/m²][mol %] A  9 1070 184 368 25 A  58 1070  51 150 33 B  68 1080  41 144 36B 100 1080  40 119 33 B 121 1080  37 116 34 C 132 1010  35 107 33 C 1501010  31  87 32 C 165 1010  30  66 27 Z 167  980  32  60 24 Z 173  980 37  47 17 Z 178  980  38  43 15

Region A extended up to diameter 58 mm and was deposited with atemperature of 1070° C.

The amount of chlorosilane per unit silicon area was 368 to 150 kg/h/m²with a molar concentration in the carrier gas (H₂) of 25 to 33%.

Region A had a compact morphology without holes and inclusions. Theporosity was less than 0.01.

Between 58 mm and 68 mm, there is a fluid transition from region A toregion B.

Region B followed on from region A over a diameter of 68 to 121 mm at atemperature of 1080° C.

The amount of chlorosilane per unit silicon area was 144 to 116 kg/h/m²with a molar concentration in the carrier gas (H₂) of 36 to 34%.

Region B had a highly porous morphology with holes and gaps. Theporosity was 0.11.

Between 121 mm and 132 mm, there is a fluid transition from region B toregion C.

Region C runs over a diameter region from 132 mm up to 165 mm.

The temperature for this was 1010° C.

The amount of chlorosilane per unit silicon area was 107 to 66 kg/h/m²with a molar concentration in the carrier gas (H₂) of 33 to 27%.

Region C had a compact morphology with few holes and gaps. The porositywas 0.05.

Between 165 mm and 167 mm, there is a fluid transition from region C tolayer Z.

Layer Z is the outermost layer, which extends from a diameter of greaterthan 167 mm up to the target rod diameter of 178 mm.

The rod temperature in the course of deposition of layer Z was 980° C.

The amount of chlorosilane per unit silicon area was 60 to 43 kg/h/m²with a molar concentration in the carrier gas (H₂) of 24 to 15%.

Z had a very compact morphology without visible holes and gaps. Theporosity was 0.01.

The rods produced in example 1 were crushed, sorted and dedusted as inthe comparative example, analogously to EP 2 423 163 A1.

A sample comprising 20 chunks of different size was selected randomlyand analyzed with regard to porosity and (if present) radius ofcurvature of the outer face.

Table 4 shows the results of these measurements.

For some chunks, it was possible to determine the radius of curvature ofthe outer face thereof. This allows conclusion of the diameter of thepolycrystalline silicon rod (here: 180 mm).

The porosities of the chunks are in the range from 0 (compact) up to0.18 (highly porous from region B).

The polysilicon chunks from example 1 were also examined with regard tothe performance thereof in crystal pulling.

For 20 rods obtained according to example 1, a mean dislocation-freelength of 98% was found.

TABLE 4 Radius of curvature of the Chunk Mass [g] Porosity outer face(if present) [mm] #1  215.7 0.01 89.6 #2   49.9 0.02 — #3   17.3 0.03 —#4  102.7 0.00 88.1 #5  114.4 0.05 — #6  217.9 0.11 — #7  150.2 0.12 —#8  160.7 0.03 — #9   22.1 0.00 89.4 #10  35.0 0.05 — #11 156.2 0.10 —#12 120.4 0.01 88.7 #13  80.5 0.08 — #14  11.3 0.02 88.2 #15 173.7 0.0088.0 #16 193.6 0.11 — #17  11.0 0.09 — #18 189.3 0.13 — #19  37.1 0.18 —#20  67.0 0.01 89.5

Example 2

For the second inventive example, a deposition plant as specified in thecomparative example was likewise utilized. As in the comparativeexample, deposition was effected with trichlorosilane and hydrogen.

Table 5 shows the process conditions.

TABLE 5 H₂ [m³ (STP)/ TCS [kg/ Conc. Layer D [mm] T [° C.] h/m²] h/m²][mol %] A  9 1070 184 368 25 A  58 1070  51 150 33 B  68 1080  41 144 36B 100 1080  40 119 33 B 132 1080  35 107 33 C 141 1010  33  97 33 C 2011010  20  49 29

In contrast to example 1, no external, very compact layer Z wasdeposited here.

Core region A extended up to diameter 58 mm and was deposited with atemperature of 1070° C.

The amount of chlorosilane per unit silicon area was 368 to 150 kg/h/m²with a molar concentration in the carrier gas (H₂) of 25 to 33%.

The morphology of core region A was compact without holes andinclusions. The porosity is less than 0.01.

Between 58 mm and 68 mm, there is a fluid transition from region A toregion B.

Region B followed on from layer A with a diameter of 68 to 132 mm at atemperature of 1080° C.

The amount of chlorosilane per unit silicon area was 107 to 144 kg/h/m²with a molar concentration in the carrier gas (H₂) of 33 to 36%.

The morphology of region B was highly porous with holes and gaps. Theporosity was 0.11.

Between 132 mm and 141 mm, there is a fluid transition from region B toregion C.

Region C is the final layer with a diameter of greater than or equal to141 mm up to the target rod diameter of 201 mm.

The temperature for this was 1010° C.

The amount of chlorosilane per unit silicon area was 55 to 97 kg/h/m²with a molar concentration in the carrier gas (H₂) of 30 to 33%.

The morphology of region C is compact, with few holes and gaps. Theporosity was 0.03.

The silicon rods produced were—as described, for example, in U.S. Pat.No. 8,074,905—crushed into chunks and sorted into chunks of size classfrom about 0.5 mm to greater than 45 mm. They were subsequentlysubjected to a wet-chemical cleaning operation as described in EP 0 905796 B1.

This gives surface metal concentrations as described in.

A sample comprising 20 chunks of different size was selected randomlyand analyzed with regard to porosity and (if present) radius ofcurvature of the outer face.

Table 6 shows the results of these measurements.

For some chunks, it was possible to determine the radius of curvature ofthe outer face thereof. This allows conclusion of the diameter of thepolycrystalline silicon rod (here: 201 mm).

The porosities of the chunks are in the range from 0.01 (not veryporous) up to 0.15 (highly porous from region B).

The polysilicon chunks from example 2 were also examined with regard tothe performance thereof in crystal pulling.

For 20 rods obtained according to example 2, a mean dislocation-freelength of 97% was found.

TABLE 6 Radius of curvature of Chunk Mass [g] Porosity the outer face[mm] #1   86.2 0.02 102.9 #2   92.4 0.08 — #3  138.6 0.10 — #4  137.80.01 101.1 #5  235.9 0.02 103.0 #6  236.8 0.02 — #7  223.7 0.08 — #8 208.9 0.09 — #9  195.2 0.01 — #10 178.4 0.02 — #11  40.9 0.01 102.8 #12118.9 0.14 — #13 182.1 0.15 — #14 230.2 0.13 — #15  59.4 0.09 — #16135.8 0.02  97.7 #17  54.7 0.10 — #18 224.0 0.00  97.8 #19  64.0 0.02104.7 #20 224.5 0.09 —

What is claimed is:
 1. A polycrystalline silicon rod having a totaldiameter of at least 150 mm, comprising: a core A having a porosity of 0to less than 0.01, which is around a thin rod; a region B around core A;and a region C around region B, wherein regions B and C differ inporosity by a factor of 1.7 to 23, and region C is less porous thanregion B.
 2. The polycrystalline silicon rod as claimed in claim 1,wherein core A extends up to a diameter of not more than 60 mm.
 3. Thepolycrystalline silicon rod as claimed in claim 1, wherein region B hasa porosity of 0.06 to 0.23 and extends over a region of 15% of the totaldiameter to a maximum of 90% of the total diameter.
 4. Thepolycrystalline silicon rod as claimed in claim 1, wherein region C hasa porosity of 0.01 to 0.1 and extends over a region of at least 50% ofthe total diameter to a maximum of 100% of the total diameter, region Chaving a lower porosity than region B.
 5. The polycrystalline siliconrod as claimed in claim 1, further comprising a layer Z around region C,which has a porosity of 0 to less than 0.01 and extends over a region ofat least 90% of the total diameter to 100% of the total diameter.
 6. Thepolycrystalline silicon rod as claimed in claim 2, wherein region B hasa porosity of 0.06 to 0.23 and extends over a region of 15% of the totaldiameter to a maximum of 90% of the total diameter.
 7. Thepolycrystalline silicon rod as claimed in claim 6, wherein region C hasa porosity of 0.01 to 0.1 and extends over a region of at least 50% ofthe total diameter to a maximum of 100% of the total diameter, region Chaving a lower porosity than region B.
 8. The polycrystalline siliconrod as claimed in claim 7, further comprising a layer Z around region C,which has a porosity of 0 to less than 0.01 and extends over a region ofat least 90% of the total diameter to 100% of the total diameter.
 9. Aprocess for producing polycrystalline silicon chunks by comminuting apolycrystalline silicon rod as claimed in claim
 8. 10. Thepolycrystalline silicon chunks produced by a process as claimed in claim9, comprising chunks with different porosities and chunks having acurved surface with a radius of curvature of at least 75 mm.
 11. Aprocess for producing polycrystalline silicon chunks by comminuting apolycrystalline silicon rod as claimed in claim
 1. 12. Thepolycrystalline silicon chunks produced by a process as claimed in claim11, comprising chunks with different porosities and chunks having acurved surface with a radius of curvature of at least 75 mm.
 13. Aprocess for producing polycrystalline silicon rods by introducing areaction gas comprising a silicon-containing component into a reactor,as a result of which polycrystalline silicon is deposited on thin rodsup to a target rod diameter, said process comprising: (a) in a firststep, depositing a core A having a porosity of 0 to less than 0.01 ontoeach of the thin rods up to a rod diameter of 15-60 mm, the rodtemperature being 1000° C. to 1150° C., the concentration of thesilicon-containing component in the reaction gas being 20 to 60 mol %and a feed rate of the silicon-containing component being 100 to 550kg/h per 1 m² of rod surface area; (b) in a second step, beginning at arod diameter of at least 10% up to a rod diameter of at most 90% of thetarget rod diameter, depositing a layer B having a porosity of 0.06 to0.23 onto core A, the rod temperature being 1030° C. to 1130° C., theconcentration of the silicon-containing component in the reaction gasbeing 20 to 40 mol % and the feed rate of the silicon-containingcomponent being 80 to 200 kg/h per 1 m² of rod surface area; and (c) ina third step, beginning at a rod diameter of at least 50% up to a roddiameter of at most 100% of the target rod diameter, depositing a layerC having a porosity of 0.01 to 0.1 onto layer B, the rod temperaturebeing 960° C. to 1030° C. and being at least 20° C. lower than the rodtemperature during the second step, the concentration of thesilicon-containing component in the reaction gas being 15 to 35 mol %,and the feed rate of the silicon-containing component being 10 to 130kg/h per 1 m² of rod surface area.
 14. The process as claimed in claim13, further comprising a fourth step, wherein beginning at a roddiameter of at least 90% up to a rod diameter of at most 100% of thetarget rod diameter, a layer Z having a porosity of 0 to less than 0.01is deposited onto layer C, the rod temperature being 930° C. to 1000° C.and being at least 20° C. lower than the rod temperature during thethird step, the concentration of the silicon-containing component in thereaction gas being 3 to 30 mol %, and the feed rate of thesilicon-containing component being 6 to 60 kg/h per 1 m² of rod surfacearea.